Structural and magnetic properties of (${\mathrm{CH}}_3$${)}_4$NNi(${\mathrm{NO}}_2$${)}_3$: A Haldane-gap system

We report investigations on the structure and the magnetic properties of (${\mathrm{CH}}_3$ ${)}_4$NNi(${\mathrm{NO}}_2$ ${)}_3$. From extended x-ray-absorption fine structure and x-ray-diffraction experiments, we show that (${\mathrm{CH}}_3$ ${)}_4$NNi(${\mathrm{NO}}_2$ ${)}_3$ has a chainlike structure of Ni(II) ions coupled via three ${\mathrm{NO}}_2$ anions. High-temperature susceptibility is consistent with one-dimensional antiferromagnetism with J/k=-12 K and g=2.25. Below 10 K, a strong decrease of susceptibility and proton relaxation rate is observed, in agreement with the Haldane prediction of a singlet ground state for the S=1, one-dimensional Heisenberg antiferromagnet. From the magnetization-versus-field data at low temperature, the Haldane gap is estimated to 4.5 K, close to the expected value 0.41 ‖J‖ for the S=1, one-dimensional Heisenberg antiferromagnet.